drm/vc4: Add "Broadcast RGB" connector property

Some HDMI monitors do not abide by the full or limited
(16-235) range RGB flags in the AVI infoframe. This can
result in images looking washed out (if given limited and
interpreting as full), or detail disappearing at the extremes
(given full and interpreting as limited).

Copy the Intel i915 driver's approach of adding an override
property ("Broadcast RGB") to force one mode or the other.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>